update mail/thunderbird to 78.2.2

See https://support.mozilla.org/en-US/kb/new-thunderbird-78 for major
changes in Thunderbird 78, the release announcement in
https://blog.thunderbird.net/2020/07/whats-new-in-thunderbird-78/ and
https://blog.thunderbird.net/2020/09/openpgp-in-thunderbird-78/ info
for OpenPGP users. Also check UPDATING 20200918.
Current release notes are in
https://www.thunderbird.net/en-US/thunderbird/78.2.2/releasenotes/

PR: 249346
Submitted by: jbeich